These are very significant studies. The results by Lee and colleagues show that extra TREM2 gene dosage is beneficial for memory deficits and AD-associated neuropathology in AD mouse models. This is the first step for us to be able, perhaps, to use increased levels of TREM2 therapeutically. A lot needs to be done, but this is the first hint that it may be possible to prevent or delay the onset of AD, or to modulate the severity of the disease, by augmenting the levels of TREM2. When we identify a new gene contributing to a disease this is the ultimate goal, and it’s great to see the amazing work that has been done by many groups trying to understand how TREM2 is functionally acting in AD and other neurodegenerative diseases.
The authors have also performed transcriptomic profiling to identify genes dependent on TREM2 gene dosage. This work helps us geneticists to focus on specific genes, so it works both ways.
The study by Zhao and colleagues reminds me of a cartoon diagram we put together after confirming TREM2 p.R47H as a risk factor for AD, where we speculated what we thought was happening at the cellular level in AD patients harboring TREM2 variants. The finding that TREM2 directly binds to Aβ oligomers may be the missing initial step in that process. One aspect we included in the diagram was the possibility of other risk genes also having effects in microglia and inflammatory activation. Now the authors raise the possibility of TREM2 mediating Aβ catabolism through proteasomal degradation pathways, which could bring together other risk genes known to have functions at this level.
These are really exciting days, with increasing numbers of very interesting and informative studies on the function of TREM2 in the context of Alzheimer’s disease. Following an earlier study by the Colonna lab (Song et al., 2018), Lee and colleagues developed a human TREM2 overexpression mouse model using a BAC-transgenic approach. Importantly, the authors designed their mouse model in a way to only overexpress TREM2, excluding confounding effects by inactivating the adjacent TREM-like genes.
It has to be highlighted that this study further argues in favor of a beneficial rather than detrimental role of TREM2, especially in the early phases of amyloid plaque deposition. Additionally, the study provides a large set of data to further strengthen the role of TREM2 in modulating the phagocytic capacity of microglia, as has been shown by many groups, including ours (Takahashi et al., 2005; Hsieh et al., 2009; Kleinberger et al., 2014; Xiang et al., 2016).
These data might also indicate that the increase of soluble TREM2 in human patients approximately five years before symptom onset (Suárez-Calvet et al., 2016; Suárez-Calvet et al., 2016) represents a beneficial response of microglia to clear amyloid plaques. Based on the consistent data that TREM2 loss of function reduced clustering of microglia around plaques and the strong upregulation of TREM2 in plaque-associated microglia (Frank et al., 2008; Jay et al., 2015) one was tempted to speculate that overexpression of TREM2 would increase plaque-associated microglia. Surprisingly, this is not the case, and even a reduced number of Iba1-positive microglia are clustered around plaques. While the number of Iba1+ microglia is reduced, the number of microglia with increased expression of phagocytic markers (e.g. CD68) is increased around amyloid plaques, calling for attention to include additional markers besides Iba1 in evaluating plaque-associated microglia in future studies.
Lee and colleagues further nicely show that overexpression of human TREM2 reduces neuritic dystrophy and finally also improves cognitive outcome, as measured by the contextual fear-conditioning test. As it is known that TREM2 is processed by regulated intramembrane proteolysis, releasing its ectodomain into the extracellular space (Kleinberger et al., 2014; Wunderlich et al., 2013), it is more than tempting to speculate that sTREM2 has a non-cell autonomous function. This will remain a major question to be answered in future studies.
First evidence for a non-cell autonomous function of sTREM2 was presented earlier by Song et al., who similarly used a BAC-transgenic approach demonstrating sTREM2 staining in neurons by immunohistochemistry. Here the authors also observe hTREM2 staining on a fraction of Aβ plaques, leaving open the question whether Aβ is yet another ligand for TREM2.
The study by Zhao and colleagues kicks in using a panel of in-vitro assays to demonstrate binding of oligomerized Aβ to human TREM2. Especially the data showing that Trem2 regulates electrophysiological changes in microglia upon stimulation with oligomeric Aβ are very interesting. Whether these effects are directly caused by the absence of Trem2 and hence a reduced binding of oligomeric Aβ, or due to a general locking of Trem2-deficient microglia in their homeostatic state (Mazaheri et al., 2017) requires further study.
Overall, the presented studies point toward a beneficial role of TREM2 in early stages of amyloid deposition and strongly argue for increasing efforts to find ways to modulate TREM2 as a potential therapeutic strategy in neurodegenerative diseases like Alzheimer’s.
References:
Song WM, Joshita S, Zhou Y, Ulland TK, Gilfillan S, Colonna M.
Humanized TREM2 mice reveal microglia-intrinsic and -extrinsic effects of R47H polymorphism.
J Exp Med. 2018 Mar 5;215(3):745-760. Epub 2018 Jan 10
PubMed.
Takahashi K, Rochford CD, Neumann H.
Clearance of apoptotic neurons without inflammation by microglial triggering receptor expressed on myeloid cells-2.
J Exp Med. 2005 Feb 21;201(4):647-57.
PubMed.
Hsieh CL, Koike M, Spusta SC, Niemi EC, Yenari M, Nakamura MC, Seaman WE.
A role for TREM2 ligands in the phagocytosis of apoptotic neuronal cells by microglia.
J Neurochem. 2009 May;109(4):1144-56. Epub 2009 Mar 19
PubMed.
Kleinberger G, Yamanishi Y, Suárez-Calvet M, Czirr E, Lohmann E, Cuyvers E, Struyfs H, Pettkus N, Wenninger-Weinzierl A, Mazaheri F, Tahirovic S, Lleó A, Alcolea D, Fortea J, Willem M, Lammich S, Molinuevo JL, Sánchez-Valle R, Antonell A, Ramirez A, Heneka MT, Sleegers K, van der Zee J, Martin JJ, Engelborghs S, Demirtas-Tatlidede A, Zetterberg H, Van Broeckhoven C, Gurvit H, Wyss-Coray T, Hardy J, Colonna M, Haass C.
TREM2 mutations implicated in neurodegeneration impair cell surface transport and phagocytosis.
Sci Transl Med. 2014 Jul 2;6(243):243ra86.
PubMed.
Xiang X, Werner G, Bohrmann B, Liesz A, Mazaheri F, Capell A, Feederle R, Knuesel I, Kleinberger G, Haass C.
TREM2 deficiency reduces the efficacy of immunotherapeutic amyloid clearance.
EMBO Mol Med. 2016 Sep 1;8(9):992-1004.
PubMed.
Suárez-Calvet M, Kleinberger G, Araque Caballero MÁ, Brendel M, Rominger A, Alcolea D, Fortea J, Lleó A, Blesa R, Gispert JD, Sánchez-Valle R, Antonell A, Rami L, Molinuevo JL, Brosseron F, Traschütz A, Heneka MT, Struyfs H, Engelborghs S, Sleegers K, Van Broeckhoven C, Zetterberg H, Nellgård B, Blennow K, Crispin A, Ewers M, Haass C.
sTREM2 cerebrospinal fluid levels are a potential biomarker for microglia activity in early-stage Alzheimer's disease and associate with neuronal injury markers.
EMBO Mol Med. 2016 May 2;8(5):466-76.
PubMed.
Suárez-Calvet M, Araque Caballero MÁ, Kleinberger G, Bateman RJ, Fagan AM, Morris JC, Levin J, Danek A, Ewers M, Haass C, Dominantly Inherited Alzheimer Network.
Early changes in CSF sTREM2 in dominantly inherited Alzheimer's disease occur after amyloid deposition and neuronal injury.
Sci Transl Med. 2016 Dec 14;8(369):369ra178.
PubMed.
Frank S, Burbach GJ, Bonin M, Walter M, Streit W, Bechmann I, Deller T.
TREM2 is upregulated in amyloid plaque-associated microglia in aged APP23 transgenic mice.
Glia. 2008 Oct;56(13):1438-47.
PubMed.
Jay TR, Miller CM, Cheng PJ, Graham LC, Bemiller S, Broihier ML, Xu G, Margevicius D, Karlo JC, Sousa GL, Cotleur AC, Butovsky O, Bekris L, Staugaitis SM, Leverenz JB, Pimplikar SW, Landreth GE, Howell GR, Ransohoff RM, Lamb BT.
TREM2 deficiency eliminates TREM2+ inflammatory macrophages and ameliorates pathology in Alzheimer's disease mouse models.
J Exp Med. 2015 Mar 9;212(3):287-95. Epub 2015 Mar 2
PubMed.
Wunderlich P, Glebov K, Kemmerling N, Tien NT, Neumann H, Walter J.
Sequential proteolytic processing of the triggering receptor expressed on myeloid cells-2 (TREM2) by ectodomain shedding and γ-secretase dependent intramembranous cleavage.
J Biol Chem. 2013 Nov 15;288(46):33027-36.
PubMed.
Mazaheri F, Snaidero N, Kleinberger G, Madore C, Daria A, Werner G, Krasemann S, Capell A, Trümbach D, Wurst W, Brunner B, Bultmann S, Tahirovic S, Kerschensteiner M, Misgeld T, Butovsky O, Haass C.
TREM2 deficiency impairs chemotaxis and microglial responses to neuronal injury.
EMBO Rep. 2017 Jul;18(7):1186-1198. Epub 2017 May 8
PubMed.
It is encouraging that elevating TREM2 gene expression resulted in favorable reductions in neuritic dystrophy. This suggests that there is sufficient bandwidth in microglia to augment TREM2 function as a means of modulating the immune response to amyloid pathology. The suggestion that Trem2 overexpression modifies the gene expression profile of plaque-associated microglia is intriguing, and may portend unexpected phenotypes if TREM2 is overexpressed in models of tauopathy. Given the robust expression of microglial ApoE observed in the Krasemann and Keren-Shaul studies, I would be curious to know if TREM2 overexpression affected ApoE in 5xFAD mice.
The paper by Lee et al. is a momentous step forward for understanding how TREM2 function governs AD-related pathology. Genetic approaches in humans and mice had collectively shown that partial or complete loss of TREM2 function enhanced Alzheimer's risk, but evidence for a protective effect of augmented TREM2 function was yet minimal.
Carrasquillo et al. (2017) reported that an SNP associated with higher expression levels of TREM2 and TREML1 also associated with reduced AD risk, which was suggestive, but the Lee et al. paper is the first to demonstrate a causal link between TREM2 gain of function and mitigation of amyloid-driven AD pathology in vivo.
An interesting question is whether the protective effects exerted by human TREM2 expression were simply a quantitative effect of increased gene dosage, or whether there were qualitative differences in the ability of human versus murine TREM2 to elicit a more beneficial microglial response. Indeed, certain microglial phenotypes known to depend on murine TREM2 expression, such as congregation around plaque or decreases in microglial process length and branch number, were partially allayed, not further heightened, by human TREM2 expression. However, other interpretations involving dynamic interplay between components of the microglial response that are directly vs. indirectly TREM2-dependent could also explain these results.
Zhao et al. proposed the unique and interesting finding that TREM2 and Aβ directly interact. If we assume that no impurities/co-purifying factors in the TREM2-Fc protein preps were involved, then the direct interaction deserves further experimental support since Aβ42 is a notoriously sticky peptide with 26 aliphatic and/or bulky hydrophobic residues. In our experience, the TREM2 extracellular domain is also sticky, with several surface-exposed hydrophobic residues.
Given these concerns, the paper would have been stronger if the authors supported the idea of direct TREM2/Aβ interaction in their cell-based and in vivo experiments. In these settings, the TREM2-dependent effects of Aβ treatment could have occurred through indirect interactions—notably, through lipoproteins which may convey Aβ to microglia and enhance TREM2-dependent uptake, as reported by colleagues here at Genentech (Yeh et al., 2016).
It would be nice to know whether the reported effects of Aβ in primary microglial and BV-2 cultures are observed in the absence of lipoproteins, perhaps by using serum-free media and ApoE knockout cells. This question is pertinent given the recent reports of Krasemann/Butovsky and Ulrich/Holtzman, indicating that the normal, TREM2-dependent microglial response to Aβ requires ApoE. If the direct interaction of Aβ with TREM2 is truly key for the microglial response to injected Aβ, the response should still occur to some degree in ApoE knockout mice, so hopefully this idea will be tested in the future.
References:
Carrasquillo MM, Allen M, Burgess JD, Wang X, Strickland SL, Aryal S, Siuda J, Kachadoorian ML, Medway C, Younkin CS, Nair A, Wang C, Chanana P, Serie D, Nguyen T, Lincoln S, Malphrus KG, Morgan K, Golde TE, Price ND, White CC, De Jager PL, Bennett DA, Asmann YW, Crook JE, Petersen RC, Graff-Radford NR, Dickson DW, Younkin SG, Ertekin-Taner N.
A candidate regulatory variant at the TREM gene cluster associates with decreased Alzheimer's disease risk and increased TREML1 and TREM2 brain gene expression.
Alzheimers Dement. 2017 Jun;13(6):663-673. Epub 2016 Dec 8
PubMed.
Yeh FL, Wang Y, Tom I, Gonzalez LC, Sheng M.
TREM2 Binds to Apolipoproteins, Including APOE and CLU/APOJ, and Thereby Facilitates Uptake of Amyloid-Beta by Microglia.
Neuron. 2016 Jul 20;91(2):328-40.
PubMed.
The paper by Zhao and colleagues reports that Trem2 is a receptor for Aβ, joining a long list of cell surface Aβ-binding proteins on microglia. The authors argue that Trem2 preferentially interacts with Aβ oligomers, with Kd’s in the nM range. A point of interest is that the effect of the disease-linked R47H/R62H variants on binding is analogous to that reported for other putative Trem2 ligands. Importantly, they investigated whether Aβ stimulated intracellular signaling, however, the magnitude of the phosphorylation of Syk was quite modest and studies of this nature lack a clear positive control. Perhaps one of the most interesting findings was an Aβ-stimulated Trem2-dependent depolarization of the microglia due to induction of potassium channel activity—however, the biological significance of this finding is unclear.
They found that Trem2 is necessary for efficient Aβ degradation, but curiously, the catabolism of Aβ was due principally to proteosomal degradation—which contravenes the prevailing view that internalized Aβ is trafficked principally to lysosomes for degradation. Trem2-null mice do not exhibit much change in Aβ peptide levels, and thus it is not obvious how these findings relate to the situation in vivo.
A significant finding is that injected Aβ oligomers robustly induce caspase 3 levels in microglia in vivo in a TREM2-dependent manner. I don’t know of a precedent for this effect and Aβ peptides typically do not kill microglia.
Yang and colleagues have reported the effect of human Trem2 expression in the 5XFAD mice using BAC-TREM2 transgenic mice. This is an extensive, well-designed and -executed study. This work complements a recent publication from the Colonna lab, also using BAC transgenics (Song et al., 2018). They found an approximate 30 percent decrease in filamentous plaque area. This is in contrast to Song et al., who did not observe a change in plaque burden.
The primary focus of this study was on longitudinal transcriptome analysis and this is a strength of this paper. Surprisingly, the BAC-Trem2-expressing mice exhibited a rather modest number of differentially expressed genes in the 5XFAD genotypes. Perhaps the most important outcome of this study was the demonstration that Trem2 overexpression suppressed expression of microglial genes previously linked to disease phenotypes.
Lee et al. report that there are fewer plaque-associated microglia in the BAC-expressing mice with altered morphology, but these cells elaborate greater numbers of longer processes and have enhanced phagocytic activity. This finding is counterintuitive and differs from Song et al., who observed significant increase in microglial density. Importantly, the BAC-Trem2-expressing mice exhibit improved cognition in a fear conditioning assay.
Overall, this paper reinforces the primary themes that have evolved in investigating Trem2 function but provides considerable detail to our understanding of the gene dose dependent effects of Trem2 expression in a mouse model of AD.
Comments
Van Andel Institute
These are very significant studies. The results by Lee and colleagues show that extra TREM2 gene dosage is beneficial for memory deficits and AD-associated neuropathology in AD mouse models. This is the first step for us to be able, perhaps, to use increased levels of TREM2 therapeutically. A lot needs to be done, but this is the first hint that it may be possible to prevent or delay the onset of AD, or to modulate the severity of the disease, by augmenting the levels of TREM2. When we identify a new gene contributing to a disease this is the ultimate goal, and it’s great to see the amazing work that has been done by many groups trying to understand how TREM2 is functionally acting in AD and other neurodegenerative diseases.
The authors have also performed transcriptomic profiling to identify genes dependent on TREM2 gene dosage. This work helps us geneticists to focus on specific genes, so it works both ways.
The study by Zhao and colleagues reminds me of a cartoon diagram we put together after confirming TREM2 p.R47H as a risk factor for AD, where we speculated what we thought was happening at the cellular level in AD patients harboring TREM2 variants. The finding that TREM2 directly binds to Aβ oligomers may be the missing initial step in that process. One aspect we included in the diagram was the possibility of other risk genes also having effects in microglia and inflammatory activation. Now the authors raise the possibility of TREM2 mediating Aβ catabolism through proteasomal degradation pathways, which could bring together other risk genes known to have functions at this level.
View all comments by Rita GuerreiroISAR Bioscience
These are really exciting days, with increasing numbers of very interesting and informative studies on the function of TREM2 in the context of Alzheimer’s disease. Following an earlier study by the Colonna lab (Song et al., 2018), Lee and colleagues developed a human TREM2 overexpression mouse model using a BAC-transgenic approach. Importantly, the authors designed their mouse model in a way to only overexpress TREM2, excluding confounding effects by inactivating the adjacent TREM-like genes.
It has to be highlighted that this study further argues in favor of a beneficial rather than detrimental role of TREM2, especially in the early phases of amyloid plaque deposition. Additionally, the study provides a large set of data to further strengthen the role of TREM2 in modulating the phagocytic capacity of microglia, as has been shown by many groups, including ours (Takahashi et al., 2005; Hsieh et al., 2009; Kleinberger et al., 2014; Xiang et al., 2016).
These data might also indicate that the increase of soluble TREM2 in human patients approximately five years before symptom onset (Suárez-Calvet et al., 2016; Suárez-Calvet et al., 2016) represents a beneficial response of microglia to clear amyloid plaques. Based on the consistent data that TREM2 loss of function reduced clustering of microglia around plaques and the strong upregulation of TREM2 in plaque-associated microglia (Frank et al., 2008; Jay et al., 2015) one was tempted to speculate that overexpression of TREM2 would increase plaque-associated microglia. Surprisingly, this is not the case, and even a reduced number of Iba1-positive microglia are clustered around plaques. While the number of Iba1+ microglia is reduced, the number of microglia with increased expression of phagocytic markers (e.g. CD68) is increased around amyloid plaques, calling for attention to include additional markers besides Iba1 in evaluating plaque-associated microglia in future studies.
Lee and colleagues further nicely show that overexpression of human TREM2 reduces neuritic dystrophy and finally also improves cognitive outcome, as measured by the contextual fear-conditioning test. As it is known that TREM2 is processed by regulated intramembrane proteolysis, releasing its ectodomain into the extracellular space (Kleinberger et al., 2014; Wunderlich et al., 2013), it is more than tempting to speculate that sTREM2 has a non-cell autonomous function. This will remain a major question to be answered in future studies.
First evidence for a non-cell autonomous function of sTREM2 was presented earlier by Song et al., who similarly used a BAC-transgenic approach demonstrating sTREM2 staining in neurons by immunohistochemistry. Here the authors also observe hTREM2 staining on a fraction of Aβ plaques, leaving open the question whether Aβ is yet another ligand for TREM2.
The study by Zhao and colleagues kicks in using a panel of in-vitro assays to demonstrate binding of oligomerized Aβ to human TREM2. Especially the data showing that Trem2 regulates electrophysiological changes in microglia upon stimulation with oligomeric Aβ are very interesting. Whether these effects are directly caused by the absence of Trem2 and hence a reduced binding of oligomeric Aβ, or due to a general locking of Trem2-deficient microglia in their homeostatic state (Mazaheri et al., 2017) requires further study.
Overall, the presented studies point toward a beneficial role of TREM2 in early stages of amyloid deposition and strongly argue for increasing efforts to find ways to modulate TREM2 as a potential therapeutic strategy in neurodegenerative diseases like Alzheimer’s.
References:
Song WM, Joshita S, Zhou Y, Ulland TK, Gilfillan S, Colonna M. Humanized TREM2 mice reveal microglia-intrinsic and -extrinsic effects of R47H polymorphism. J Exp Med. 2018 Mar 5;215(3):745-760. Epub 2018 Jan 10 PubMed.
Takahashi K, Rochford CD, Neumann H. Clearance of apoptotic neurons without inflammation by microglial triggering receptor expressed on myeloid cells-2. J Exp Med. 2005 Feb 21;201(4):647-57. PubMed.
Hsieh CL, Koike M, Spusta SC, Niemi EC, Yenari M, Nakamura MC, Seaman WE. A role for TREM2 ligands in the phagocytosis of apoptotic neuronal cells by microglia. J Neurochem. 2009 May;109(4):1144-56. Epub 2009 Mar 19 PubMed.
Kleinberger G, Yamanishi Y, Suárez-Calvet M, Czirr E, Lohmann E, Cuyvers E, Struyfs H, Pettkus N, Wenninger-Weinzierl A, Mazaheri F, Tahirovic S, Lleó A, Alcolea D, Fortea J, Willem M, Lammich S, Molinuevo JL, Sánchez-Valle R, Antonell A, Ramirez A, Heneka MT, Sleegers K, van der Zee J, Martin JJ, Engelborghs S, Demirtas-Tatlidede A, Zetterberg H, Van Broeckhoven C, Gurvit H, Wyss-Coray T, Hardy J, Colonna M, Haass C. TREM2 mutations implicated in neurodegeneration impair cell surface transport and phagocytosis. Sci Transl Med. 2014 Jul 2;6(243):243ra86. PubMed.
Xiang X, Werner G, Bohrmann B, Liesz A, Mazaheri F, Capell A, Feederle R, Knuesel I, Kleinberger G, Haass C. TREM2 deficiency reduces the efficacy of immunotherapeutic amyloid clearance. EMBO Mol Med. 2016 Sep 1;8(9):992-1004. PubMed.
Suárez-Calvet M, Kleinberger G, Araque Caballero MÁ, Brendel M, Rominger A, Alcolea D, Fortea J, Lleó A, Blesa R, Gispert JD, Sánchez-Valle R, Antonell A, Rami L, Molinuevo JL, Brosseron F, Traschütz A, Heneka MT, Struyfs H, Engelborghs S, Sleegers K, Van Broeckhoven C, Zetterberg H, Nellgård B, Blennow K, Crispin A, Ewers M, Haass C. sTREM2 cerebrospinal fluid levels are a potential biomarker for microglia activity in early-stage Alzheimer's disease and associate with neuronal injury markers. EMBO Mol Med. 2016 May 2;8(5):466-76. PubMed.
Suárez-Calvet M, Araque Caballero MÁ, Kleinberger G, Bateman RJ, Fagan AM, Morris JC, Levin J, Danek A, Ewers M, Haass C, Dominantly Inherited Alzheimer Network. Early changes in CSF sTREM2 in dominantly inherited Alzheimer's disease occur after amyloid deposition and neuronal injury. Sci Transl Med. 2016 Dec 14;8(369):369ra178. PubMed.
Frank S, Burbach GJ, Bonin M, Walter M, Streit W, Bechmann I, Deller T. TREM2 is upregulated in amyloid plaque-associated microglia in aged APP23 transgenic mice. Glia. 2008 Oct;56(13):1438-47. PubMed.
Jay TR, Miller CM, Cheng PJ, Graham LC, Bemiller S, Broihier ML, Xu G, Margevicius D, Karlo JC, Sousa GL, Cotleur AC, Butovsky O, Bekris L, Staugaitis SM, Leverenz JB, Pimplikar SW, Landreth GE, Howell GR, Ransohoff RM, Lamb BT. TREM2 deficiency eliminates TREM2+ inflammatory macrophages and ameliorates pathology in Alzheimer's disease mouse models. J Exp Med. 2015 Mar 9;212(3):287-95. Epub 2015 Mar 2 PubMed.
Wunderlich P, Glebov K, Kemmerling N, Tien NT, Neumann H, Walter J. Sequential proteolytic processing of the triggering receptor expressed on myeloid cells-2 (TREM2) by ectodomain shedding and γ-secretase dependent intramembranous cleavage. J Biol Chem. 2013 Nov 15;288(46):33027-36. PubMed.
Mazaheri F, Snaidero N, Kleinberger G, Madore C, Daria A, Werner G, Krasemann S, Capell A, Trümbach D, Wurst W, Brunner B, Bultmann S, Tahirovic S, Kerschensteiner M, Misgeld T, Butovsky O, Haass C. TREM2 deficiency impairs chemotaxis and microglial responses to neuronal injury. EMBO Rep. 2017 Jul;18(7):1186-1198. Epub 2017 May 8 PubMed.
View all comments by Gernot KleinbergerWashington University
It is encouraging that elevating TREM2 gene expression resulted in favorable reductions in neuritic dystrophy. This suggests that there is sufficient bandwidth in microglia to augment TREM2 function as a means of modulating the immune response to amyloid pathology. The suggestion that Trem2 overexpression modifies the gene expression profile of plaque-associated microglia is intriguing, and may portend unexpected phenotypes if TREM2 is overexpressed in models of tauopathy. Given the robust expression of microglial ApoE observed in the Krasemann and Keren-Shaul studies, I would be curious to know if TREM2 overexpression affected ApoE in 5xFAD mice.
View all comments by Jason UlrichBrigham Young University
The paper by Lee et al. is a momentous step forward for understanding how TREM2 function governs AD-related pathology. Genetic approaches in humans and mice had collectively shown that partial or complete loss of TREM2 function enhanced Alzheimer's risk, but evidence for a protective effect of augmented TREM2 function was yet minimal.
Carrasquillo et al. (2017) reported that an SNP associated with higher expression levels of TREM2 and TREML1 also associated with reduced AD risk, which was suggestive, but the Lee et al. paper is the first to demonstrate a causal link between TREM2 gain of function and mitigation of amyloid-driven AD pathology in vivo.
An interesting question is whether the protective effects exerted by human TREM2 expression were simply a quantitative effect of increased gene dosage, or whether there were qualitative differences in the ability of human versus murine TREM2 to elicit a more beneficial microglial response. Indeed, certain microglial phenotypes known to depend on murine TREM2 expression, such as congregation around plaque or decreases in microglial process length and branch number, were partially allayed, not further heightened, by human TREM2 expression. However, other interpretations involving dynamic interplay between components of the microglial response that are directly vs. indirectly TREM2-dependent could also explain these results.
Zhao et al. proposed the unique and interesting finding that TREM2 and Aβ directly interact. If we assume that no impurities/co-purifying factors in the TREM2-Fc protein preps were involved, then the direct interaction deserves further experimental support since Aβ42 is a notoriously sticky peptide with 26 aliphatic and/or bulky hydrophobic residues. In our experience, the TREM2 extracellular domain is also sticky, with several surface-exposed hydrophobic residues.
Given these concerns, the paper would have been stronger if the authors supported the idea of direct TREM2/Aβ interaction in their cell-based and in vivo experiments. In these settings, the TREM2-dependent effects of Aβ treatment could have occurred through indirect interactions—notably, through lipoproteins which may convey Aβ to microglia and enhance TREM2-dependent uptake, as reported by colleagues here at Genentech (Yeh et al., 2016).
It would be nice to know whether the reported effects of Aβ in primary microglial and BV-2 cultures are observed in the absence of lipoproteins, perhaps by using serum-free media and ApoE knockout cells. This question is pertinent given the recent reports of Krasemann/Butovsky and Ulrich/Holtzman, indicating that the normal, TREM2-dependent microglial response to Aβ requires ApoE. If the direct interaction of Aβ with TREM2 is truly key for the microglial response to injected Aβ, the response should still occur to some degree in ApoE knockout mice, so hopefully this idea will be tested in the future.
References:
Carrasquillo MM, Allen M, Burgess JD, Wang X, Strickland SL, Aryal S, Siuda J, Kachadoorian ML, Medway C, Younkin CS, Nair A, Wang C, Chanana P, Serie D, Nguyen T, Lincoln S, Malphrus KG, Morgan K, Golde TE, Price ND, White CC, De Jager PL, Bennett DA, Asmann YW, Crook JE, Petersen RC, Graff-Radford NR, Dickson DW, Younkin SG, Ertekin-Taner N. A candidate regulatory variant at the TREM gene cluster associates with decreased Alzheimer's disease risk and increased TREML1 and TREM2 brain gene expression. Alzheimers Dement. 2017 Jun;13(6):663-673. Epub 2016 Dec 8 PubMed.
Yeh FL, Wang Y, Tom I, Gonzalez LC, Sheng M. TREM2 Binds to Apolipoproteins, Including APOE and CLU/APOJ, and Thereby Facilitates Uptake of Amyloid-Beta by Microglia. Neuron. 2016 Jul 20;91(2):328-40. PubMed.
View all comments by David HansenIndiana University School of Medicine
The paper by Zhao and colleagues reports that Trem2 is a receptor for Aβ, joining a long list of cell surface Aβ-binding proteins on microglia. The authors argue that Trem2 preferentially interacts with Aβ oligomers, with Kd’s in the nM range. A point of interest is that the effect of the disease-linked R47H/R62H variants on binding is analogous to that reported for other putative Trem2 ligands. Importantly, they investigated whether Aβ stimulated intracellular signaling, however, the magnitude of the phosphorylation of Syk was quite modest and studies of this nature lack a clear positive control. Perhaps one of the most interesting findings was an Aβ-stimulated Trem2-dependent depolarization of the microglia due to induction of potassium channel activity—however, the biological significance of this finding is unclear.
They found that Trem2 is necessary for efficient Aβ degradation, but curiously, the catabolism of Aβ was due principally to proteosomal degradation—which contravenes the prevailing view that internalized Aβ is trafficked principally to lysosomes for degradation. Trem2-null mice do not exhibit much change in Aβ peptide levels, and thus it is not obvious how these findings relate to the situation in vivo.
A significant finding is that injected Aβ oligomers robustly induce caspase 3 levels in microglia in vivo in a TREM2-dependent manner. I don’t know of a precedent for this effect and Aβ peptides typically do not kill microglia.
Yang and colleagues have reported the effect of human Trem2 expression in the 5XFAD mice using BAC-TREM2 transgenic mice. This is an extensive, well-designed and -executed study. This work complements a recent publication from the Colonna lab, also using BAC transgenics (Song et al., 2018). They found an approximate 30 percent decrease in filamentous plaque area. This is in contrast to Song et al., who did not observe a change in plaque burden.
The primary focus of this study was on longitudinal transcriptome analysis and this is a strength of this paper. Surprisingly, the BAC-Trem2-expressing mice exhibited a rather modest number of differentially expressed genes in the 5XFAD genotypes. Perhaps the most important outcome of this study was the demonstration that Trem2 overexpression suppressed expression of microglial genes previously linked to disease phenotypes.
Lee et al. report that there are fewer plaque-associated microglia in the BAC-expressing mice with altered morphology, but these cells elaborate greater numbers of longer processes and have enhanced phagocytic activity. This finding is counterintuitive and differs from Song et al., who observed significant increase in microglial density. Importantly, the BAC-Trem2-expressing mice exhibit improved cognition in a fear conditioning assay.
Overall, this paper reinforces the primary themes that have evolved in investigating Trem2 function but provides considerable detail to our understanding of the gene dose dependent effects of Trem2 expression in a mouse model of AD.
View all comments by Gary LandrethMake a Comment
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